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Vitamin D in the Winter

By Maryhelen Mackin Chadwick

If you exercise outside during the winter, can you get enough sun exposure to manufacture the vitamin D you need?

According to a 2009 article published by the Harvard School of Public Health, if you live north of the line connecting San Francisco to Philadelphia, the answer is probably no.

Keri Gans, RD, a spokesperson for the American Dietetic Association, says geography and time of year are just a few of many factors that affect vitamin D levels. Others are diet, age, skin pigmentation, time of day you exercise and the type of clothing you wear.

The New York City-based nutritionist says when the sun’s UVB rays hit the skin, a reaction takes place that manufactures vitamin D. The optimal time for this to take place is midday — in the summer. During the winter, the sun’s UVB rays aren’t strong enough to cause the needed reaction.

“Even if you stood outside naked in Ohio at midday in the winter, you would still need to supplement your intake of vitamin D,” Gans says.

If you think you can get enough vitamin D indoors by drinking milk (with or without your clothes on), you may want to think again.

One glass of milk is fortified with 100 IUs vitamin D. The current government recommended daily allowance is 200-400 IUs. That upper end is roughly equal to one quart of milk. If you drink a quart of milk each day, you may meet the requirement — until the recommendation changes to 1,000 to 2,000 IUs a day —which is what Gans and fellow ADA spokesperson Roberta Anding, expect to happen soon, because vitamin D is gaining attention for other effects it has in the body.

Anding, who is director of sports nutrition at Baylor College of Medicine and Texas Children’s Hospital, says most people think of vitamin D in relation to metabolism and bone strength because it helps bones absorb calcium and phosphorous. New studies are showing it is an intricate hormone (yes, vitamin D is actually a hormone) for balancing our body’s functions including maintaining muscle mass, preventing illness, combating chronic pain and improving athletic performance.

“Think of vitamin D as the conductor of your body’s orchestra. It tells the cells what to do, when to do it and how to function,” Anding says, “Now as more focus is turning to vitamin D, studies are suggesting a greater prevalence of deficiencies.”

Symptoms of vitamin D deficiency are bone pain and muscle weakness, but you can be deficient without any noticeable symptoms. A blood test will show if you have a vitamin D deficiency. Anding says paying approximately $100 for a blood test is comparable to other expenses you incur to improve your performance as an adventure athlete.

Both Gans and Anding recommend taking a vitamin D supplement year round because it’s too difficult to ensure you are consistently getting enough naturally. Because it is a fat-soluble vitamin, it is important to take the supplement with a meal that contains some fat.

But, Gans says, “That does not give you permission to take it with a bowl of ice cream.”